Wafer inspection system including a laser triangulation sensor

ABSTRACT

One example of an inspection system includes a laser, a magnification changer, and a first camera. The laser projects a line onto a wafer to be inspected. The magnification changer includes a plurality of selectable lenses of different magnification. The first camera images the line projected onto the wafer and outputs three-dimensional line data indicating the height of features of the wafer. Each lens of the magnification changer provides the same nominal focal plane position of the first camera with respect to the wafer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Application No. 16/620,317,filed Dec. 6, 2019, which claims the benefit of InternationalApplication No. PCT/US2018/036573, filed Dec. 8, 2018, which claims thebenefit of U.S. Provisional Pat. Application No. 62/516,701, filed Jun.8, 2017, all of which are incorporated herein by reference.

BACKGROUND

Semiconductor wafers may be inspected using inspection systems tomeasure features of the wafer for quality control. It is advantageous toincrease throughput, improve accuracy, increase dynamic range, improvereliability, and reduce the cost of inspection systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating one example of an inspectionsystem including a laser triangulation sensor.

FIG. 2 illustrates one example of a camera for the laser triangulationsensor.

FIG. 3 is a block diagram illustrating one example of interface/controlboard connections for the laser triangulation sensor.

FIG. 4 illustrates example calibration blocks for calibrating the lasertriangulation sensor.

FIGS. 5A and 5B illustrate one example of a wafer being inspected usingthe laser triangulation sensor.

FIG. 6 illustrates one example of a laser line viewed by the camera ofthe laser triangulation sensor.

FIGS. 7A-7D illustrates the laser line as viewed by the camera andsample frames obtained by the camera.

FIG. 8 illustrates one example of using two cameras to inspect a wafer.

FIG. 9 illustrates one example of using two lasers to inspect a wafer.

FIG. 10 illustrates one example of inspecting a wafer from the reverseside of the wafer.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific examples in which the disclosure may bepracticed. It is to be understood that other examples may be utilizedand structural or logical changes may be made without departing from thescope of the present disclosure. The following detailed description,therefore, is not to be taken in a limiting sense, and the scope of thepresent disclosure is defined by the appended claims. It is to beunderstood that features of the various examples described herein may becombined, in part or whole, with each other, unless specifically notedotherwise.

Disclosed herein is a system and method of laser triangulation for waferinspection. A laser line generator may project a line onto the surfaceof a wafer. The laser line may be imaged onto a three-dimensional (3D)camera by microscope optics. The 3D camera may acquire two-dimensional(2D) images of the laser line, convert the 2D images into 3D lines in afield programmable gate array (FPGA) processing board, and output the 3Dlines to a frontside computer via a universal serial bus (USB) (e.g.,USB3.0) interface.

FIG. 1 is a block diagram illustrating one example of an inspectionsystem 100 including a laser triangulation sensor. The inspection system100 may include a sensor head 102, a stage 104 to support a wafer 106 tobe inspected, a trigger board 108, and a frontside computer 110. Thetrigger board 108 may be communicatively coupled to the sensor head 102via a trigger/encoder signal path 112. The frontside computer 110 may becommunicatively coupled to the sensor head 102 via a USB (e.g., USB3.0)interface 114.

The sensor head 102 may include a first 3D camera 116, an isolator 118,a specular filter/blocker 120, a magnification changer (e.g.,turret/slide) 122, a laser mount 124, and an interface/control board126. The first 3D camera 116 may include a first camera enclosure 128, amount 130, a tube 132, and a lens 134. The magnification changer 122 mayinclude a plurality of objective lenses 136 ₁ to 136 ₃ of differentmagnification (e.g., 2x, 10x, and 5x). The magnification changer 122 mayprovide the same nominal focal plane position of the first camera 116with respect to the wafer 106 for each of the objective lenses 136 ₁ to136 ₃ by shimming the objective lenses.

The laser mount 124 may include a laser 138, an attenuator 140, a mirror142, and a quarter wave plate 144. The laser 138 may generate a laserline 146, which may be attenuated by the attenuator 140, reflected bythe mirror 142 and passed through the quarter wave plate 144 forprojection onto the wafer 106. The attenuator 140, mirror 142, andquarter wave plate 144 between the laser 138 and the wafer 106 mayprovide a circularly polarized laser line on the surface of the wafer106 at an angle of 45 degrees to the wafer normal. The laser mount 124may allow for the following adjustments: translation in Z to get thefocal point on the objective lens center axis, rotation about Z to getthe line parallel to the tool Y axis, rotation about X to get the linecenter on the objective lens center axis or to flatten the field of thelaser, and rotation about Y to get the plane on the objective lensnominal working distance.

The quarter wave plate 144 may convert the naturally linear polarizedlight out of the laser 138 to circularly polarized light. The attenuator140 may be a neutral density type fixed attenuator to achieve a slightreduction in laser power. The mirror 142 may be a turning mirror toredirect the laser beam 146 to the wafer 106. The mirror 142 may includea rotation about Y adjustment to get the plane passing through thenominal tool point. Rotation about Z may also be used to get the linecenter on the objective lens center axis or to flatten the field of thelaser 138. The laser line 146 projected onto the surface of the wafer106 is reflected back toward the first camera 116 through themagnification changer 122, the specular filter/blocker 120, the isolator118, and the camera lens 134.

FIG. 2 illustrates one example of a camera 200. In one example, thecamera 200 may be used for camera 116 of FIG. 1 . The camera 200 mayinclude 3D camera boards 202 and 204, an image sensor 206, a cameraenclosure 208, a camera mount/tube 210, and a camera lens 212. The imagesensor 206 may be a mono complementary metal-oxide- semiconductor (CMOS)sensor with an electronic shutter. The camera 200 may include an imageprocessing mode that is configurable for 3D or 2D. The maximum framerate for 3D mode may be 57,000 fps or more for a 1280 × 50 window size,29,500 fps or more for a 1280 × 100 window size, or 3,500 fps or morefor a 1280 × 864 window size. The maximum frame rate for 2D mode may bethe lesser of the 3D mode rate listed above and (200 MB/s / (framesize)). X windowing may be size configurable from 1 to 1280. The startposition may be configurable from 0-1279. The frame rate should increaseinverse proportional to size between 1280 and 1184. Size below 1184 maybe achieved by data skip with no frame rate benefit. Y windowing may besize configurable in quanta of 4. The start position may be configurablefrom 0 to 860. The frame rate should change inverse proportional tosize. The output format of the camera 200 in 3D mode may be 32-bit (16-bit Z and 16-bit Intensity); and the output format in 2D mode may beconfigurable for 8-bit (stored as 8-bit) or 10-bit (stored as 16-bit).

The camera 200 may be mounted with the long dimension of the imagesensor 206 parallel to the Y axis. The camera 200 may be mounted in sucha way that a splitter and second camera 148 (FIG. 1 ) may be used ifdesired. The enclosure 208 and mount 210 may provide the followingadjustments: mount translation in X (e.g., adjustable with gaugeblocks), mount translation in Z/Focus (e.g., adjustable with gaugeblocks), and mount rotation about Z. These adjustments may primarily beused to compensate for image sensor position/rotation errors withrespect to the enclosure 208, however, they may also be used to removeresidual errors after laser line position adjustment.

The receiver section of the camera mount 210 may enable reconfigurationfor camera lenses with focal lengths in the range of 148 mm to 295 mm.Referring back to FIG. 1 , the isolator 118 may work in conjunction withthe quarter wave plate 144 to block laser light from double bouncesbetween two bumps or between the wafer 106 surface and a bump. Theisolator 118 may consist of a preassembled quarter wave plate and linearpolarizer. The isolator 118 may be mounted between the camera lens 134and objective lens 136 ₁ to 136 ₃ with the linear polarizer facing thecamera lens.

An automated specular filter/blocker 120 may be used when inspectingdiffuse reflective surfaces such as pre-reflow bumps at lowmagnification. The specular filter/blocker 120 may be located close tothe objective lens aperture. The specular filter/blocker 120 may includea plurality of selectable filters and/or blockers. The specularfilter/blocker 120 may include a wheel with flag/phase positioning. Lowmoment of inertia (MOI) and friction may allow for a small power steppermotor to position blockers or filters quickly and accurately. Themultiple blockers may have different sizes and may have different shapesdepending on what spatial blocking would provide the cleanest signal(similar to micro inspection or scattering tools that block all lightnot related to the signals that are of interest). A filter may include aFourier filter. Liquid crystal display (LCD) as well as solid blockingmaterial (e.g., Vantablack) may be used. The specular filter/blocker 120may be controlled automatically using recipes and may include a means todetect which position the filter/blocker is in and/or to detect if thefilter/blocker is not fully in one of the positions.

A cylindrical lens 150 or receiver defocus may be used. In otherexamples, cylindrical lens 150 is not used. As the radius of the top ofa mirror like spherical bump becomes small compared to the laser linewidth, the data can exhibit a stair step effect. This effect is at itsworst when the spherical surface spreads the light far beyond theobjective lens numerical aperture (NA) causing a diffraction limitedline to be formed on the image sensor. When the line width is less thanone pixel, the stair step effect is easily visible. The centroid erroras a function of laser image size for a Gaussian shape should be kept afactor of two below the 1/16^(th) subpixel resolution, which comes to 3%of a pixel. Accordingly, the image width should be at least 1.5 pixelsor larger to keep the centroid error below 3%. A weak cylindrical lensthat increases the camera lens focal length (FL) in the X direction maybe used to accomplish this. The camera may also be moved closer to thecamera lens to accomplish this, however, this also defocuses Y which maybe desirable or undesirable depending on the amount of defocusing andthe feature being inspected. The camera focus adjustment may haveadditional travel in the negative Z direction to accommodate defocusingto increase the objective lens NA diffraction limited spot size to atleast three pixels.

The magnification changer 122 of the sensor head 102 may automaticallyswitch between two or three objective lenses 136 ₁ to 136 ₃ via recipecontrol. The magnification changer 122 does not cause the distancebetween the 2D and laser triangulation optical center lines to increase.The magnification changer 122 may include a means to detect which lenspositon the magnification changer is in. If the magnification changer122 is not fully in a lens positon, then the magnification changer doesnot report as being in any positon.

The magnification changer 122 may support multiple interchangeableobjective lenses of the same family. For example, the objective lensesmay include any suitable combination of the following: 2x, 3x, 5x, 7.5x,10x, or 20x lenses. The objective lenses may be manually swapped in thefield with only configuration file changes and recalibration. Opticaladjustments should not be required. If the parfocal distance varies toomuch between objectives, then custom spacers may be used to adjust thedistance. In this case, a master 10x objective with nominal spacer maybe used in manufacturing so that all production 2x, 3x, 5x, and 10xobjectives may be spaced to the ideal parfocal distance.

FIG. 3 is a block diagram illustrating one example of interface/controlboard 126 connections for the laser triangulation sensor. Theelectro-mechanical devices in the sensor head 102 may be controlled bythe interface/control board 126 located in the sensor head.Communications from the frontside computer 110 (FIG. 1 ) to theinterface/control board 126 may be through the 3D camera 116 via the USBinterface 114 and serial peripheral interface (SPI) interfaces 160. Thisapproach may minimize the number of cables and centralize the controlfunctions, which may reduce cost and improve reliability andserviceability.

The interface/control board 126 may include a micro controller chip(MCU) with SPI, general purpose input/output (GPIO), analog to digitalconverters (ADCs), and digital to analog converters (DACs). Theinterface/control board 126 may support at least one laser (e.g., laser138) and two 3D cameras (e.g., 3D cameras 116 and 148). Theinterface/control board 126 may distribute power from power path 162 topower path 164 to the 3D camera(s) 116 and/or 148, laser(s) 138 andother devices as necessary. The interface/control board 126 may controlthe magnification changer 122 (e.g., turret/slide) through acommunication path 166, the specular filter/blocker 120 through acommunication path 170, and the on/off and output power of the laser(s)138 through a communication path 168.

The interface/control board 126 may receive the RS422 trigger andencoder signals through signal path 112 and convert them to single endedTTL signals for the 3D camera(s). The interface/control board 126 mayoutput the trigger signals to the 3D camera(s) through a signal path 172and output the encoder signals to the 3D camera(s) through a signal path174. In one example where two 3D cameras are used, one camera mayreceive odd numbered triggers and the other camera may receive evennumbered triggers.

The interface/control board 126 may support trigger buffering logic,which queues up triggers when the triggers are coming in too fast duringacceleration overshoot and velocity ripple peaks and then catches upduring velocity ripple valleys. The maximum queue depth may beconfigured. With the XY stage, trigger buffering may allow velocitysafety margins as small a 0.3% to be used while falling behind by nomore than 1 trigger. The trigger buffering logic may use the triggeroutput signal from the camera through signal path 172 to decide when thenext trigger can be sent. In other examples, the trigger buffering logicmay be implemented by trigger board 108 or by 3D camera 116.

FIG. 4 illustrates example calibration blocks 300 and 302 forcalibrating the laser triangulation sensor. A rectangle may be used forZ calibration, laser power calibration, Z origin calibration, and todetermine the normal vector of the calibration block to compensatecalibration in the event the calibration block was not perfectly level.A rectangle may also be used for Z vs. Y flatness checks, Z noise levelchecks, and scan pass to scan pass Z vs. Y uniformity checks. Therectangle boundaries may be used for checking stray light and bright todark or dark to bright transition response.

A star pattern may be used for Y calibration and XY origin calibration.A star pattern may also be used for profiling the laser throughout theentire Y field of view (FOV) and Z FOV and for checking and/or adjustingthe calibration block slope and rotation. In one example, the rectangleand star features accommodate a 2 mm Z FOV and an 8 mm Y FOV.

FIGS. 5A and 5B illustrate one example of a wafer 320 being inspected.FIG. 5A illustrates a side view of wafer 320, and FIG. 5B illustrates atop view of wafer 320. In one example, the laser line 322 is projectedonto the wafer 320 at 45 degrees as indicated by arrow 324 and thereflection of the laser line 322 is imaged by the camera as indicated at326. The wafer 320 is moved via the stage in the direction indicated byarrow 328 for each image as indicated for example by the dashed lines.Multiple features (e.g., bumps) 330 may be measured simultaneously.

FIG. 6 illustrates one example of a laser line 360 viewed by the camera.The position of the line in the direction indicated at 362 sensed by thecamera indicates the height of the features of the wafer. An entire lineof data is detected at once.

FIG. 7A illustrates one example of a laser line 370 as viewed by thecamera and FIGS. 7B-7D illustrate sample frames obtained by the camera.The camera allows unwanted data to be filtered from bump top centroidcalculation, which improves accuracy.

FIG. 8 illustrates one example of using two cameras to inspect a wafer400. Two cameras may be used to increase the speed of inspection. In oneexample, the first camera (indicated by pattern 402) and the secondcamera (indicated by pattern 404) are coupled to the optical path by abeam splitter such that the two cameras of the inspection systemsubstantially share the same field of view. The stage upon which a waferis supported may be moved in the direction indicated by arrow 406 at arate relative to the field of view of the cameras such that each camera,when operated alternately, captures successive fields of view (i.e.,interleaving) that cover substantially the entire surface of the waferthat is to be imaged. The velocity of the stage may be correlated to therate at which the two cameras capture images of the field of view andthe size of the field of view of the cameras.

FIG. 9 illustrates one example of using two lasers to inspect a wafer.This example includes a first laser 420, a second laser 422, a mirror424, and an attenuator 426. The first laser 420 generates a laser line428, which passes through mirror 424 and is attenuated by the attenuator426. The second laser 422 generates a laser line 430, which is reflectedby mirror 424 and is attenuated by the attenuator 426. A second cameracapable of capturing images in a range of wavelengths to which the waferis at least partially transparent may be used with the second laser 422that emits light in the range of wavelengths viewable by the secondcamera. In this case, the second laser 422 emits light in the range ofwavelengths to which the wafer is at least partially transparent.

FIG. 10 illustrates one example of inspecting a wafer 450 from thereverse side 452 of the wafer. A laser line 454 may be projected by asecond laser onto one or more features 456 through the reverse side 452of the wafer 450. In this case, the second camera captures an image ofthe projected line from the second laser and outputs three-dimensionalline data indicating the height of features of the wafer. The featuresmay be selected from a group consisting of vias and trenches.

Although specific examples have been illustrated and described herein, avariety of alternate and/or equivalent implementations may besubstituted for the specific examples shown and described withoutdeparting from the scope of the present disclosure. This application isintended to cover any adaptations or variations of the specific examplesdiscussed herein. Therefore, it is intended that this disclosure belimited only by the claims and the equivalents thereof.

What is claimed is:
 1. An inspection system comprising: a stage for supporting a wafer to be inspected; a laser line generator configured and arranged relative to the stage to project a laser line onto the wafer; and a camera system arranged relative to the stage to image the laser line as reflected from the wafer, wherein the camera system is configured to: acquire two-dimensional images of the reflected laser line, convert the two-dimensional images into three-dimensional lines formatted to indicate a height of features of the wafer, and output the three-dimensional lines to indicate the height of features of the wafer. 